1. Field of the Invention
The present invention relates to a gypsum composition for denture investment used for the preparation of dental restorations such as complete or partial plate dentures with the use of resin providing a denture base material.
2. Description of the Prior Art
The dental profession has heretofore utilized as an important material a gypsum composition for denture investment serving as a mould material, when preparing dentures applied in the mouth. However, little proposal has been made of gypsum compositions for denture investment used as mould materials for holding artificial teeth. For that reason, use has generally been made of .beta.-Gypsum hemihydrate (.beta.-CaSO.sub.4.1/2H.sub.2 O) exclusively used for building or ornamental purposes. However this .beta.-Gypsum hemihydrate (.beta.-CaSO.sub.4.1/2H.sub.2 O) has a low wet-compressive strength 100 kgf/cm.sup.2, that is considered to be the strength of the set gypsum per se. Therefore, when dough like resin is packed under pressure in a set mass covering artificial teeth, the thin gypsum portion surrounding the denture is readily damaged. In addition, due to the fact that the set gypsum shows a setting expansion of as high as 0.25%, the fitness of the plate having the denture invested therein drops upon the polymerization of resin and the artificial teeth shift to such an extent that restoration should be carried out for a very long period of time so as to reproduce the occlusal balance inherent in the patient. Futhermore, upon allowed to stand at room temperature, the setting time of .beta.-Gypsum hemihydrate (.beta.-CaSO.sub.4.1/2H.sub.2 O) is delayed due to the absorption of moisture, thus posing problems in connection with storage stability.
To make up for the disadvantages of .beta.-Gypsum hemihydrate (.beta.-CaSO.sub.4.1/2H.sub.2 O), in some cases, use may be made of .alpha.-Gypsum hemihydrate (.alpha.-CaSO.sub.4.1/2H.sub.2 O) that is dental model gypsum. Owing to the fact that the .alpha.-Gypsum hemihydrate (.alpha.-CaSO.sub.4.1/2H.sub.2 O) has originally been developed as a model material, it shows a wet-compressive strength on the order of 450 kgf/cm.sup.2 upon set, that is quadruple as large as that of .beta.-Gypsum hemihydrate (.beta.-CaSO.sub.4.1/2H.sub.2 O), with no substantial risk of being damaged. However, since there is little lowering of the compressive strength of the set gypsum upon the polymerization of denture base resin by heating, an extreme difficulty is encountered in removal of the denture after the resin has been polymerized by heating.
Clinical requirements strongly demanded for the gypsum compositions for denture investment are now that:
1. They should have a sufficient strength after setting, provided that at the time of removal of the denture upon the polymerization of resin by heating, they have a deteriorated strength to facilitate removal of the denture.
2. They should have a small setting expansion so that the dimensional accuracy of the denture is excellent upon the polymerization of resin by heating. And,
3. They should excel in storage stability.